We see the pictures that show how close this nation came to another school massacre. We told you last night about the bullets and bombs amassed by a student at the University of Central Florida in Orlando and tonight. We tracked down the roommate credited with making the 911 call that may have saved the campus. ABC's -- tuchman is there -- Tonight the first video from police cameras. Tracking down he would be killer body got a camera showing the moment -- with assault rifles. Discovered the body of -- all receive the -- along with his arsenal mass murder. In the background that fire alarm police say the shooter set it off. -- -- to get students in the hallway and slaughter them. The suspected shooter pulled the fire -- down went back into his shooting here. Then he grabbed his -- apparently he went after one of his roommates who then slid into the bathroom and that roommate rob about -- -- called 911. -- -- -- Arms and got a -- now. We spoke to him today you -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- Police say -- -- helped prevent a massacre. Police say -- Cooper and left behind a check to play I think every step of the attack as he did each thing actually scratched -- off on the list. So that was and the -- stops with -- firearm he had amassed an arsenal -- delegates -- and massive clips -- we learned tonight it could have been much worse people packages were found in the college -- What I was in the mail waiting -- the deceased. Were. Two. The 22 round magazines there was also a Blackhawk technical slaying and there was also a training -- In a short statement -- -- -- -- parents say their son was a -- What had no history of violence. Matt Gutman ABC news Orlando. And tonight Matt tells us that at this hour the motive is still a complete mystery.

This transcript has been automatically generated and may not be 100% accurate.

Now Playing: Defense for Dzhokhar Tsarnaev Gets Ready to Rest Case After Four Witnesses

Now Playing: Aaron Hernandez Trial: Patriots Owner Robert Kraft Called as a Surprise Witness

Now Playing: Nine Crew Members From Tourist Pirate Ship Rescued in Stormy Waters